# -*- coding: utf-8 -*-
"""
定义一个数据结构：栈

@author: DonghaiHan
"""

class Stack:
    
    def __init__(self,items=[]):
        self.items=items
        
        
    def pop(self):
        return self.items.pop()
    
    def push(self,v):
        self.items.append(v)
        
    def __len__(self):
        return len(self.items)
    
    def isEmpty(self):
        
        return self.items==[]
    
    def __str__(self):
        return str(self.items)
    
    
s=Stack()
for i in range(1,20,3):
    s.push(i)
    
print(len(s))
    
